Electroplating is an established process of producing a metallic coating on a surface. Such coatings may perform a protective function to prevent corrosion of the metal on which they are deposited, e.g., plating with zinc or tin (electro-galvanizing); or a decorative function, e.g., gold or silver plating; or both functions, e.g., chromium plating.
The principal of electroplating is that the coating metal is deposited from an electrolyte, typically an aqueous acid or alkaline solution, onto a target substrate or panel. The latter forms the cathode (negative electrode) while a plate of the metal to be deposited serves as the anode (positive electrode).
During a standard electroplating process, the periphery of the printed circuit board, i.e., the portions of the printed circuit board adjacent its outer edges, tends to be at a higher current density than the center of the printed circuit board. Hence, metal deposits more rapidly adjacent the periphery of the printed circuit board than at the center. The result of this is that by the time the metal has deposited at the center of the circuit board to form a desired thickness, the metal deposited adjacent the periphery is at a thickness much greater than the thickness at the center. As a result, the width of depositing metal lines may grow laterally, and the resulting plated lines near the periphery may develop a cross sectional configuration resembling a mushroom.
In U.S. Pat. No. 4,828,654, it is reported that by spacing the cathode a relatively large distance from the anode, and by making the effective size of the panel to be plated, i.e. the cathode, larger in size than the anode, there is more uniform distribution of the electroplating field. The more uniformly distributed field causes the metallic ions to be electrolytically deposited at a more uniform rate over the articles in the panel. This prior art arrangement reportedly avoids undesirable uneven plating build-up on the articles at those areas where there is a concentration of the electroplating field. It is also reported that field concentrations occur when the size of the article is smaller than the size of the anode, and results in the edges of the article experiencing a substantial greater build up of metallic ions than the center area of the article. Making the effective size of the cathode (the article to be plated) greater than the size of the anode and spacing the anode a relatively large distance from the cathode, operates to discourage the formation of areas of concentration in the electroplating field and encourages the ion transfer to become more uniform over the entire area of the cathode.
U.S. Pat. No. 4,828,654 teaches an anode used in electroplating formed by a plurality of individual anode segments which can be selectively energized to establish an effective anode size that relates to the size of the article to be electroplated, thereby establishing an electrical field of more uniform characteristics to transfer ions from the anode to the articles at a more uniform deposition rate over the whole surface of the article. By adjusting the effective size of the anode to correspond and relate to the size of the article, the non-uniform deposition rates associated with concentrated localized field reportedly are avoided, and the physical size of the electroplating apparatus can be reduced.
U.S. Pat. No. 4,933,061 teaches an electroplating apparatus for electroplating a plurality of items. The patented apparatus includes a tank having a bottom wall and side walls, adapted to hold a predetermined quantity of electrolytic plating solution. A sparger system at the bottom of the tank directs the electrolytic plating solution in an upward direction. A cathode rack supports the items to be electroplated and extends intermediate to the anode plates and upwardly from the sparger system. Strategically placed openings in the anodes and an anode screen in conjunction with the sparger system reportedly act to reduce the plating thickness variance over the rack.
In U.S. Pat. No. 5,017,275, there is disclosed an anode structure comprising a resilient anode sheet having an active anode surface, and a support sub-structure for the anode sheet. The anode sub-structure has a pre determined configuration. By flexing the anode sheet onto the anode sub-structure, so that the anode sheet conforms to the configuration of the anode sub-structure, there reportedly is provided an adequate electrical junction for substantially uniform current distribution.
A collection of the known variables which affect the electroplating process have been set out in detail in the HANDBOOK OF PRINTED CIRCUIT MANUFACTURING by Raymond H. Clark (1985). Therein it is reported that the factors which effect the electroplating process include: 1. plating pattern geography; 2. panel thickness and size of plated through holes; 3. panel boarders; 4. plating rack; 5. bath chemistry, e.g., concentration of metals and acids, concentration of organic leveling and brightening agents, concentration of contaminants; 6. bath temperature; 7. anode-cathode spacing; 8. anode current density; 9. anode depletion; 10. plating bath agitation; 11. cathode agitation; 12. rectifier consideration; and 13. the skill and experience of the plater.
The present invention provides an improved electroplating system which overcomes the aforesaid and other problems of the prior art which have resulted in less than uniform electroplating and metallic deposition, and in so doing provides substantially uniform distribution of the deposited metal, from item to item in an electroplating process.